This invention relates generally to a system for transporting or storing semisolid materials, such as grease or ground or comminuted food products, and liquid materials, such as oil or printers ink, in bulk quantities, and more particularly to a tank adapted for quickly and efficiently unloading semisolid or liquid material contained therein.
U.S. Pat. Nos. 4,721,235 and 5,114,054 (incorporated herein by reference) disclose a tank for bulk transport and storage of semisolid and liquid materials. The tank has a follower piston with a pneumatically expandable rubber seal at one end thereof for seating the piston relative to the tank while accommodating changes in interior cross-section of the tank and means, such as a plurality of pads fastened to the piston and extending radially outwardly therefrom, for preventing canting of the piston as it moves within the tank. The seal is positioned between two parallel flanges around the piston which extend radially outwardly from the piston for resisting axial movement of the seal relative to the piston.
When the tank is new its interior surface closely approximates a circular cylinder with little variance of the inside diameter of the hank. An exemplary tank has an inside tank diameter of approximately seventy inches and utilizes an air pressure of about 10-12 psig in the seal to properly expand the seal against the tank's interior surface. As the tank gets older, imperfections (such as out of round areas caused by sagging of the tank, or dents or "clings") may form resulting in increasingly larger variations in the inside diameter of the tank. To ensure that the entire surface of the tank is wiped, it may be necessary to increase air pressure in the seal to as much as 50 psig to accommodate these imperfections.
Although the increase in air pressure expands the seal to accommodate portions of the tank having increases in tank diameter, forces exerted by the seal on portions of the tank having reductions in Lank diameter are much greater than necessary to wipe the tank. These forces result in high frictional forces tending to cause the seal to wear and to scuff.
Also, because of the outward expansion of the seal, the high frictional forces, and the shape of the seal, axial movement of the piston relative to the tank may cause the crown of the seal to shift axially relative to the piston and be pinched between one of the flanges and the tank thereby resulting in an accelerated wear and shortened life of the seal and/or resulting in gouging of the seal by the flange.